This study compares Atovaquone and Azithromycin (AT/AZ) to trimethoprim- sulfamethoxazale (TMP/SMX) in the prevention of serious bacterial infection in children with HIV infection. The primary objectives of the study are to determine the comparability of each arm in the prevention of serious bacterial infections, to determine the long term safety and tolerance of these drugs and to determine if there is pharmacokinetic interaction between atovaquone and azithromycin.